The present invention claims priority from Japanese Patent Application No.9-206439 filed Jul. 31, 1997, which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a control method of a network terminal connection for allowing a mobile terminal to communicate with either one of a trunk network to which the mobile terminal is connected through a junction node serving as a communication interface and an ad hoc network constructed with a plurality of mobile terminals connected thereto without using a junction node when the mobile terminal moves between the these networks, and a construction of the mobile terminal whose communication connection is controlled thereby.
2. Description of Related Art
There are two types of networks known among recent communication networks for mobile terminals. One of them is a network called a "trunk network" in which mobile terminals are mutually connected through junction nodes such as routers as in the Internet, and the other is a network called an "ad hoc network" in which there is no junction node provided and which is temporarily constructed with only mobile terminals.
When a mobile terminal establishes a communication connection in a network of either type, it is required to (1) grant an address of the mobile terminal used in the network to which the mobile terminal is connected or an address of the network and make the address known to all as common knowledge and (2) grant an address of a mobile terminal to be communicated therewith and make it known to all.
The grant of these addresses as common knowledge is performed in the trunk network, particularly, the Internet, by a server using the Dynamic Host Configuration Protocol (DHCP) for the requirement (1) and using the Domain Name System (DNS) for the requirement (2). DHCP and DNS are described in detail in, for example, Douglas E. Comer, "Internet working with TCP/IP, Volume I: Principles, Protocol and Architecture", Third edition, 1995, Prentice-Hall Inc.
In DHCP, mobile terminal addresses and trunk network addresses are stored in a DHCP server and, when a mobile terminal is to be connected to one of the networks, the mobile terminal requests the DHCP server for information necessary for connection thereof to the network, such as a mobile terminal address or a network address. In response to the request from the mobile terminal, the DHCP server assigns a mobile terminal address to the requesting mobile terminal such that the mobile terminal address does not conflict with those of other mobile terminals and provides the assigned mobile terminal address to the requesting mobile terminal.
FIG. 1 shows a DHCP sequence between the DHCP server and mobile terminals. In FIG. 1, mobile terminals #1 and #2 requesting connections to the network send DHCP requests by broadcast transmission. A DHCP server which receives the requests sends DHCP acknowledgement (DHCP ACK's) containing information of the address to be assigned to these mobile terminals #1 and #2, respectively. In the example shown in FIG. 1, the DHCP server sends DHCP ACK containing an address #1 to the mobile terminal #1 and DHCP ACK containing an address #2 to the mobile terminal #2.
FIG. 2 is a flowchart showing a sequence of procedures when a mobile terminal moves between trunk networks. When a mobile terminal enters from a trunk network into another network, the mobile terminal searches for the existence of communication media such an as Ethernet or wireless local area network (Wireless LAN), etc., to which the mobile station can be connected (Step S1). When there is no such communication media (S1: No), the mobile terminal can not be connected for communication and the procedure is ended (Step 2). On the contrary, when there is such communication media to which the mobile station can be connected (S1: Yes), the mobile terminal confirms the existence of the trunk network as the communication media (Step S3). When there is no trunk network (Step S3: No), the mobile terminal can not be connected for communication and the procedure is ended (Step S2). When there is a trunk network (Step S3: Yes), the mobile terminal performs a connection to the trunk network (Step S4).
The detection of the trunk network and the connection thereto can be realized by utilizing a technique called "Agent Discovery, Registration" disclosed in, for example, C. Perkins, "IP Mobility Support", Request for Comments, RFC2002, October 1996. In the disclosed technique, in order to support a mobile terminal moving between trunk networks, a node called a "Mobility Agent" is introduced in the trunk network.
In this case, in order to support the movement of the mobile terminal, a Home Address which is used regardless of the movement of the mobile terminal and a "Care of" Address which is used in a trunk network to which the mobile terminal is moved are assigned to the mobile terminal and a correspondence between these two addresses is managed by a Home Agent. The Home Agent temporarily receives a packet designated to the Home Address and transfers the packet to the mobile terminal at the destination thereof on the basis of a Care of Address corresponding to the Home Address.
In order to allow the mobile terminal to confirm the existence of the trunk network, a method called "Agent Discovery" is used. In this method, the Mobility Agent periodically transmits a message containing its own IP (Internet Protocol) address and an information (net mask) of length of a network portion in its IP address to broadcast. A value obtained by net-masking the IP address of the mobility agent coincides with an identifier of the trunk network. Such message is called a "trunk network identifier advertisement message". The mobile terminal determines whether or not there is a trunk network on the communication media, depending upon whether or not it can receive the trunk network identifier advertisement message within a predetermined time. The mobile terminal may also transmit a "trunk network identifier request message" requesting a transmission of the trunk network identifier advertisement message.
When the mobile terminal confirms the existence of the trunk network, the mobile terminal acquires a care-of address to be used in that trunk network by using DHCP and provides the care-of address to the home agent. Thus, the correspondence between the home address and the care-of addresses is registered in the home agent and the movement processing is completed.
On the other hand, in the case of an ad hoc network which is constructed with only mobile terminals, there is no server such as a DHCP server or mobility agent, etc. Therefore, it is impossible to connect a mobile terminal to the ad hoc network through the same procedure as that used in the trunk network.